The invention relates to a metal gasket installed between two members, such as a cylinder head and a cylinder block, of an internal combustion engine to seal therebetween. More specifically, the invention relates to a metal gasket formed of a single metal base plate or a plurality of metal base plates coated with a coating.
When joint surfaces between the cylinder head and the cylinder block (cylinder body) of the engine of the automobile are sealed, the cylinder head gasket is installed therebetween to seal combustion gas, cooling water and the like.
In view of the demand for a light weight and low cost of the engine, a structure of the cylinder head gasket has been shifted to a simple type formed of a single or two metal base plates from a laminated type having a number of metal plates. Due to a few constituent plates, only limited types of materials can be used.
In the laminated type metal plate gasket, a sealing member, such as a bead, grommet and shim, can be combined if necessary and provided in double for effective sealing. However, in the single or two plate gasket, a type and a number of sealing members are limited, so that a simplified sealing device has to be used. Moreover, the area available for the sealing device is limited with reduction of an engine size.
As shown in FIG. 5, the cylinder head gasket is formed to have a shape of an engine member such as a cylinder block. The cylinder head gasket includes holes 2 for cylinder bores (hereinafter referred to simply as “cylinder bore 2”); fluid holes 3, 4 for circulating cooling water and engine oil (hereinafter referred to simply as “fluid holes 3, 4”); and bolt holes 5 for tightening bolts and the like. Also, the sealing devices, such as beads 12, 13, with respect to the respective holes 2, 3 to be sealed, are provided.
In one gasket for sealing between the engine members, different sealing properties are required to seal the different holes. At the cylinder bore 2, it is required to seal the combustion gas with a high temperature and a high pressure, while a fluid with a relatively low temperature and a low pressure is required to be sealed at the fluid hole 3.
An arc shape bead 12 with a narrow width, which is formed of a material with elasticity to fit the uneven surface, is provided around the cylinder bore 2. Also, the cylinder bore 2 is surrounded by the bolt holes 5. Bolts are inserted into the bolt holes 5 to tighten the cylinder head and the cylinder block by applying a large pressure.
On the other hand, the bead 13 is provided around the fluid hole 3 to seal the same. Depending on a structure of the engine, the fluid hole 3 may be formed outside the area surrounded by the bolt holes 5. In this case, pressing forces by the tightening bolts are applied on only one side of the fluid hole 3, so that the tightening forces become small. To solve the problem, a higher bead, a narrower bead, or a pointed bead has been used.
Further, to improve the fitting property of the metal surface abutting against the bead and the gasket, a coating has been used.
For example, in the gasket disclosed in Japanese Utility Model Publication (JITSUKAIHEI) No. 2-16861, a bead in a belt shape is partially provided and, at the same time, a coated elastic film is disposed on both surfaces of the metal base plate. In this case, the thickness of the coated elastic film provided on the bead projection side is made thicker than that on the other side, so that a strong tightening pressure applied to the top portion of the bead is absorbed by the thick coated elastic film to thereby keep the same tightening surface pressure as that on the other side. Thus, a good surface pressure balance can be maintained, and a high torque holding ability and durability can be obtained.
However, in the gasket, since a large sealing surface pressure is applied to the top portion of the bead, when the gasket is used for a long time, the coated elastic film covering the bead top portion is pushed to cause the flow or wearing-out. Thus, the sealing surface pressure is substantially reduced to cause the torque down.
Also, in a gasket disclosed in Japanese Utility Model Registration No. 2605613, a rubber-like elastic material is coated on both surfaces of a supply member formed of a metal plate and provided with a bead. In the gasket, a rubber hardness of the coating applied on the bead projection side is made higher than that on the other side to suppress the flow of the coating at the top portion of the bead. To make the rubber hardness on the other side lower, a surface pressure per unit area is held at a low value to thereby balance the sealing properties on both surfaces.
However, in the gasket, even if the sealing surface pressure is lowered, by the coating of the lower rubber hardness applied on the other side, as a whole, the sealing surface pressure on the bead top portion becomes partially extremely higher than that of the peripheries of the bead by the coating having the high rubber hardness applied on the bead projection side. Thus, there has been a problem wherein the surface pressure applied to the coating on the bead top portion becomes high, and the flow and torque-down, which results in a substantial lowering of the surface pressure, can not be prevented.
A gasket disclosed in Japanese Patent Publication (TOKUKAIHEI) No. 10-103523, includes the first coating layer provided on the bead projection side of a metal plate except for the bead top portion, and the second coating layer provided on the other side of the metal plate. The first coating layer is prevented from flowing at the bead top portion to thereby suppress the torque-down, which is a reduction of the sealing surface pressure at the bead portion with the passage of time.
However, since the coating layer is not applied on the bead top portion, in case the small unevenness, such as a tool mark, is present on the surface of the engine member abutting against the bead top portion, the unevenness can not be absorbed. Therefore, there has been a risk that the combustion gas, cooling water, oil and the like may slightly leak.
In view of the above problems, the present invention has been made. An object of the invention is to provide a metal gasket, wherein in case joint surfaces between the cylinder head and the cylinder block of the engine are sealed by a metal gasket formed of a single or a plurality of metal plates, the surface pressure distribution around the bead can be properly held, and good sealing property and durability capable of suppressing the flow of the coating at the bead top portion and the torque-down brought by the flow, can be obtained.
Further objects and advantages of the invention will be apparent from the following description of the invention.